In recent years the light-emitting diode (LED) has become a commonplace device for a broad range of applications. In the visible range it may provide communication between an electronic device and the user. In the infrared range it may have broad applications for communications. It may be used in an optical isolator for decoupling an input signal from an output. In many applications it is an important desideratum that the LED emits a large amount of light for a given current.
Transparent LEDs emit light at a p-n junction within the body of semiconductor forming the LED. Light is emitted in all directions from the junction and because of the transparency of the device, light may be emitted from its four sides and its front surface. The back surface is opaque, but some light may be reflected from it and emitted through the sides or front. A portion of the front surface may be occulted by the need for an electrical contact. Thus, light may be emitted from four sides and a portion of the front.
Extracting light from an LED is not easy because of the high index of refraction of the semiconductor material which may be in the range of from about 2.9 to 4.0, depending on wavelength and material. According to Snell's law EQU sin.theta..sub.c =n.sub.s /n.sub.p
only rays that impinge on the chip surface at an angle equal to or less than .theta..sub.c will be refracted through the surface. All rays impinging at angles greater than .theta..sub.c will experience total internal reflection. In other words, only light emitted from a point source within the chip within a cone of total cone angle 2.theta..sub.c having its axis perpendicular to the face of the semiconductor chip will escape from the LED.
Assuming that the index of refraction, n.sub.c, for the semiconductor is 3.3 and the index of refraction, n.sub.p, for a transparent plastic surrounding the semiconductor is 1.5, the critical angle for total internal reflection, .theta..sub.c, is 27.degree.. Assuming that the point source of light is isotropic, we find that a fraction, f, of the light flux is within such a cone where f is given by ##EQU1## where the term within the brackets is the correction for Fresnel reflection losses. Where n.sub.c equals 3.3 and n.sub.p equals 1.5, the escape cone contains about 5.2% of the light emitted by the isotropic point source.
LED chips are usually made by a scribe and break technique, resulting in a rectangular parallelepiped where the side faces are smooth crystallographic planes intersecting the front and back surfaces.
An LED chip has six orthogonal surfaces and therefore six possible escape cones. In such a rectangular body reflected rays never change their angle of incidence. In other words, rays emitted in a direction outside of the six escape cones will always remain outside of the escape cones no matter how many reflections they experience. Such rays keep bouncing around within the LED until they eventually are absorbed.
Four of the escape cones directed at the side surfaces are unobstructed. The cone directed towards the back contact surface is partly absorbed and partly reflected towards the front. The cone directed towards the front is partly transmitted through the front and partly occulted and absorbed by the front electrical contact. As a result, in a typical transparent LED light is extracted through only about five cones or approximately 25% of the light generated by the LED is actually emitted.
When the LED is operating in air instead of in transparent plastic, emission is even poorer since the critical angle for total internal reflection is only about 16.degree. to 18.degree.. For this reason it is customary to operate LEDs embedded in transparent plastic for maximum light extraction efficiency.
By making the chip an imperfect rectangular body, for example by sawing the chip instead of cleaving it, somewhat roughened side faces may be obtained. Light is scattered from such roughened surfaces from non-escape directions into the escape cones. Some light within the escape cones may be internally reflected from a roughened surface. Further, the randomizing of the light directions requires many reflections and because of the non-negligible level of absorption, the long light path within the LED results in only a modest increase in extraction efficiency.
Taken to its ultimate or optimal configuration for light extraction, a LED would have a surface of a hemisphere so that light from a small p-n junction in its center is normal to the surface regardless of the ray direction. Such hemispherical LEDs have been built and are highly efficient, but extremely high in price because of the complex processing required. Some of such prior hemispherical LEDs were made by temporarily connecting two LEDs together base-to-base, then tumbling them in a rotating polishing mill until spherical. Alternatively, some of the prior hemispherical LEDs were made by attaching the LED chip to a dop and polishing much as one would polish a lens. The cost of LEDs made by such techniques is prohibitive for most applications.
It is, therefore, desirable to provide means for improving the efficiency of light extraction from an LED. It is also desirable that the technique be one easily implemented in manufacturing operations for LEDs without significantly decreasing the yield of LED chips from a wafer of semiconductor.